Nutrient mining generally refers to agricultural practices resulting in a negative nutrient balance: export (loss) of a nutrient is greater than import (input).

Estimates suggest that the total area under cultivation remained more or less constant (at 140-142 Mha) over the past several decades, and there are indications that the agricultural lands are gradually being diverted to accommodate increased urbanization and industrialization. It is unlikely that sizable additional area will be brought in under cultivation in the foreseeable future. Therefore, there is no other viable option than increasing crop productivity per unit area, to meet the future production goals.

Maintenance of native soil fertility in the intensively cultivated regions of the country is one of the preconditions of maintaining and improving the current crop yield levels.

Intensive cropping systems remove substantial quantities of plant nutrients from soil during continued agricultural production round the year. The basic principle of maintaining the fertility status of a soil under high intensity crop production systems is to annually replenish those nutrients that are removed from the field.

Indeed this becomes more relevant in the absence of the measures for adequate replenishment of the depleted nutrient pools through the removal of crop residues from agricultural fields (Sanyal 2014).

The term “Nutrient Mining” is used when the quantity of soil nutrients removed by a crop from an agricultural field exceeds the amount of the nutrient that is recycled back and/ or replenished to the field.

Nutrient mining causes a decline in the native soil fertility and may seriously jeopardize future food security of the country. Unfortunately, the concern for nutrient mining in Indian soils is largely limited to the scientific community and has not been integrated adequately with the crop production practices.

The nutrient mining issues concern the nutrients that are less mobile in soils and have higher potential of staying in the soil. For example, nitrogen (N) is highly mobile in the soil and has the highest probability, among the major nutrients, to be lost from the soil system through volatilization and leaching, among others.

The Indian soils, being in the sub-tropical region coupled with the preponderance of tillage practices, are rarely sufficient in N. Nitrogen is generally applied in adequate quantities to the crops, and “nitrogen mining” is not frequently discussed as crop production relies more on adequate external application through fertilizer/manure sources rather than on the native soil reserve of N. The input-output balance calculations for N, at the regional or the national scale, generally show positive balance in soils (Katyal 2001)

Nutrient Mining – Balancing “additions” and “removals”

The biggest entry on the plus side of the nutrient balance equation is the native soil reserve of a nutrient at any given point of time. The nutrients may come from several sources including parent material, irrigation water, crop residues, or as by-products of natural events. The other two significant entries on the plus side are external application of manures and fertilizer.

Although all the additions are dynamic in nature, the proportion of nutrient addition through irrigation water, crop residue, etc., to the total reserve of a nutrient in a soil is low. This is particularly true for nutrients like K. In common banking parlance, this could be considered as a “fixed deposit” that is saved for posterity. Manures and fertilizer application to soil, on the other hand, could be considered as “regular deposit” that is immediately available for use by the plants.

Maintenance of soil fertility requires that we use the “regular” deposits for crop production without significantly depleting the “fixed” deposits. The largest minus in the nutrient balance equation is the crop uptake and removal. Crop uptake and removal becomes synonymous when crop residues are largely removed from the fields along with the harvest of the economic products.

This is particularly true for most field crops grown in India as there are a large number of competitive uses of crop residues. Based on the achieved yields, crops remove nutrients from the soil and that constitutes the largest depleting factor of soil nutrients. Besides crop removal, there are other avenues of losses of nutrients from the soil such as volatilization, leaching, erosion, run-off, etc.

These losses could contribute significantly to the negative side of the nutrient balance equation under specific crop growing conditions. Volatilization of surface applied urea in calcareous soils or leaching of K in coarse textured soils are examples of typical growing environment-induced losses of nutrients from soil that could be easily rectified by modifying the management decisions.

Such nutrient input-output information could be used to develop nutrient balance information at local, regional or national scale. This may help in fertilizer application decision at the field scale to fertilizer import and distribution policies at the regional or the national scale.

Knowledge Gaps and Future Initiatives

Reciprocal Internal Efficiency

The Reciprocal Internal Efficiency (RIE) appears prominently as a major contributor to the nutrient balance equations and for determining the nutrient rates using different approaches.

The RIE has its origin in the modified QUEFTS (Quantitative Evaluation of the Fertility of Tropical Soils) model that assesses the relationship between the grain yield of rice and the corresponding nutrient accumulation as a function of the climatic yield potential and the supply of the three macronutrients.

The underlying principle is that the relationship between grain yield and nutrient accumulation may be described as a function of the climatic yield potential and the supply of the three macronutrients N, P and K. In a situation where crop growth is not limited by water

Nutrient Content in Irrigation Water

Nutrient input from irrigation water and losses through leaching features prominently in nutrient balance equations that helps estimate fertilizer requirement. Irrigation water contains essential plant nutrients, particularly K, which upon addition to soil improves soil fertility.

Presence of K in irrigation water constitutes an important source of indigenous supply of K to plants. The K input from irrigation water depends primarily on (a) K concentration in the added water and (b) the quantity of water added during the entire crop production cycle, i.e., from the onset of land preparation to harvest.

Crop Residue Management

Crop residues, in general, are parts of the plants left in the field after crops have been harvested and threshed. The recycling of crop residues can significantly add to the nutrient input in a cropping system.

In general, farmers remove crop residues from the fields for other competitive use such as animal feed, etc. This aggravates the nutrient mining from soils. Buresh et al. (2010) analyzed several scenarios in cereal systems to show the critical importance of crop residue retention in agricultural fields to maintain the nutrient balance.

A National Soil Data Repository

It goes without saying that a national portal for soil data repository is a critical requirement for assessing soil nutrient mining. Initiation to develop and maintain a soil data repository will allow tracking of soil fertility changes in intensive cropping regions over time.

Such databases are fragmented and maintained by several organizations that are not available in the public domain. Integrating such fragmented databases into one national portal will help overall assessment of the national soil resources and developing other knowledge resources, such as fertility maps for different soil nutrients at a finer scale.

Developing a national portal of soil data will strongly fit into the current initiative of generating the “Soil Health Card” for millions of farm fields in the country. The geo-referenced soil analysis data from the “Soil Health Card” initiative could be stored in the national soil data repository and would be a logical starting point for a “national soil data repository” for the posterity.

This will be an extremely valuable resource to help in research, planning and implementation of the improved agricultural practices at local, regional and country scale. From, the nutrient mining standpoint, such a repository will help reorient fertilizer management practices based on agro-climate, soil type and management practices to minimize soil nutrient mining and sustain the soil fertility levels.

Nutrient mining in agriculture cannot be avoided altogether. Indeed, different soils, under similar cropping systems and comparable management practices, will differ considerably in their inherent buffer powers to withstand the stress arising from “nutrient mining”. In other words, the degree of soil vulnerability varies (Sanyal 2014).

Multiple cropping systems and management practices adopted by farmers on numerous soil types in the country further complicate the nutrient mining scenario. Therefore, the allowable range of nutrient mining under variable climate-soil-crop-management domain needs to be assessed, at least at the regional scale.

A national effort to address the nutrient mining may go a long way to maintain the quality of our soils for the posterity and to ensure the food security of the future generations.

Dwivedi, B.S., Singh, V.K. and Kumar, V. 2011. Maximizing the rice-wheat system productivity through balance fertilizer use under Typic Ustochrept soils of Western Indo-Gangetic Plain. Journal of FarmingSystem Research and Development 17: 1-14.

Katyal, J.C. 2001. Fertilizer use situation in India. Journal of the Indian Society of Soil Science 49: 570-592.